This application is a continuation-in-part of application Ser. No. 247,920, filed Sept. 28, 1988, to issue on Dec. 5, 1989 as U.S. Pat. No. 4,885,216, which is a continuation-in-part of application Ser. No. 678,802, filed Dec. 6, 1984, now U.S. Pat. No. 4,677,035, issued June 30, 1987.
1. Field of the Invention
This invention relates, in general, to novel and improved nickel base single crystal alloys and, in particular, to such alloys having the combined properties of oxidation/corrosion resistance, high strength at elevated temperatures, and improved high solid solution strength, high precipitation strength, and a broadened processing "window" between their gamma prime solvus or solutioning temperature and their incipient melting temperature. More specifically, the present invention relates to novel and improved nickel base single crystal alloys which retain their high temperature solid solution strength and mechanical properties after prolonged or repeated exposure to elevated temperatures, the single crystal alloys being capable of being cast into desired shapes, such as turbine blades, vanes and other parts used in high temperature gas turbine engines which exhibit excellent hot corrosion resistance. Even more specifically, the present invention relates to the above type of improved and novel nickel base single crystal alloys which can be heat treated to improve stress-rupture life of such alloys coated with conventional coatings with an accompanying heat treatment to impart high temperature oxidation/sulfidation resistance thereto without the formation of deleterious phases at the alloy/coating substrate interface.
2. Description of the Prior Art
Nickel base superalloys which have been commonly used over the years to fabricate gas turbine engine components typically contain, aside from certain levels of chromium, cobalt, aluminum, titanium and refractory metals (e.g., tungsten, molybdenum, tantalum and columbium) other elements which act as grain boundary strengtheners, such as carbon, boron and zirconium. These types of gas turbine blades are most commonly formed by casting, and the process most often utilized produces parts having equiaxed non-oriented grains. Since the high temperature properties or solid solution strength of metals are generally dependent upon grain boundary properties, efforts have been made to strengthen the boundaries of these alloys by addition of carbon, boron and/or zirconium, as discussed above, or to reduce or eliminate the grain boundaries transverse to the major stress axis of the part. One method of eliminating such transverse boundaries is directional solidification as described in U.S. Pat. No. 3,260,505. The effect of directional solidification is to produce an oriented microstructure of columnar grains whose major axis is parallel to the stress axis of the part and which has minimal or no grain boundaries perpendicular to the stress axis of the part.
A further extension of this concept is the utilization of single crystal parts in gas turbine blades as, for example, described in U.S. Pat. No. 3,494,709. The obvious advantage of the single crystal blade is the complete absence of grain boundaries and therefore the absence of these potential weak areas of a metallic structure. Thus, the mechanical properties of the single crystal are completely dependent upon the inherent mechanical properties of the material. While single crystal nickel base alloys are generally known, there exists a need for such alloys having a varied combination of properties including improved mechanical strength, especially over prolonged and/or repeated exposure to elevated temperatures, improved oxidation/corrosion resistance (especially at elevated temperatures) and the ability to be cast to desired shapes, such as turbine blades and parts, which are processed at elevated temperatures above their gamma prime solvus or solution temperature but below their incipient melting temperature. If this temperature "window" is too narrow the alloy cannot be used reliably for many commercial applications.
There exists in the patent literature many examples of nickel base superalloys and nickel base single crystal superalloys, methods for their fabrication and methods for their heat treatment. Examples of some of these prior art patents and the basic technology that they describe, is as follows:
In commonly assigned U.S. Pat. No. 4,677,035 there is described nickel base single crystal alloys which are improved, with regard to their strength and corrosion properties at elevated temperatures, by the addition of certain elements in certain amounts.
In commonly assigned U.S. Pat. No. 4,885,216 there are described nickel base single crystal alloys which represent an improvement over those of said U.S. Pat. No. 4,677,035 with respect to improved retention of high strength and long term phase stability, due to the addition of certain amounts of hafnium and silicon, and optionally having greater strength at elevated temperatures and hot corrosion resistance or greater resistance to oxidation/sulfidation at elevated temperatures due to the addition of certain amounts of yttrium, lanthanum and/or manganese.
Schweizer et al., U.S. Pat. No. 4,222,794, discloses a nickel base single crystal superalloy for use at elevated temperatures having a restricted composition consisting of 4.5-6.0% chromium, 5.0-5.8% aluminum, 0.8-1.5% titanium, 1.7-2.3% molybdenum, 4.0-6.0% tungsten, 5.5-8.0% tantalum, 1.0-5.0% rhenium, 0.2-0.6% vanadium, 0-7.0% cobalt and the balance nickel This patent also discloses a method of heat treating the alloys described therein at a specific temperature range. Although the Schweizer et al patent discloses a single crystal alloy, said alloy differs chemically from the alloy of the present invention. For example, the alloy of the present invention is significantly higher in chromium content, titanium content and titanium to aluminum ratio, and does not contain rhenium and vanadium.
Gell et al., U.S. Pat. No. 4,116,723, discloses single crystal nickel base super alloys free from intentional additions of cobalt, boron, and zirconium. Gell at al. discusses the avoidance of the development in the single crystal alloys of deleterious phases after long term exposure at elevated temperatures (i.e., alloy instability), the phases being of two general types, sigma and mu. Sigma is undesirable because of its brittle nature while mu is undesirable because the phase ties up large amounts of the refractory solid solution strengtheners thus weakening the remaining alloy phases. The sigma and mu phases are termed TCP phases for topologically closed packed phases and one of their common properties is that they all contain cobalt. Gell et al. eliminates cobalt in the claimed single crystal nickel base alloys to inhibit the formation of TCP phases therein. Unexpectedly, the presence of cobalt in the single nickel base alloys of the present invention does not induce the formation of TCP phases. Also, the ratio of titanium to aluminum disclosed by Gell et al. is lower than that in the alloy of the present invention. While U.S. Pat. No. 4,116,723 relates to heat treatment of single crystal alloys, precipitation-hardened alloys having the high temperature mechanical properties of the instant invention (e.g., retention of high temperature properties after prolonged or repeated exposure to elevated temperature) are not obtained.
Duhl et al. U.S. Pat. No. 4,402,772 discloses single crystal superalloy casting compositions, and superalloy coatings applied thereto. The casting compositions consist essentially of 3.5-7% tantalum, 7.4-11% chromium, 4-6% cobalt, 0.6-1.8% titanium, 0-2.5% molybdenum, 6-12% tungsten, 4.5-6% aluminum, 0.05-0.5 hafnium and balance nickel. Other metals, such as rhenium and vanadium, are acknowledged as known superalloy additives but are excluded by Duhl et al.
Harris et al. U.S. Pat. No. 4,582,548 discloses single crystal superalloy casting compositions having no hafnium except in the form of trace amounts thereof.
Duhl et al U.S. Pat. No. 4,801,513 discloses single crystal superalloy casting compositions consisting essentially of 5-18% chromium, at least 5% of aluminum or titanium, at least 5% of molybdenum, tungsten, tantalum, columbium, rhenium or silicon, and the balance nickel. Rhenium is an optional ingredient and its function is not disclosed.
There are many patents disclosing various single crystal superalloy compositions having different specific ingredients and proportions to provide specific properties of the types required by the particular application in which the superalloys are to be used. Some ingredients, and/or proportions thereof, impart properties which may be undesirable for certain applications, and certain amounts of additive materials or metals may not impart the same properties to one composition as they do to another, because of the different properties of the base alloys.
Among the disadvantages of the prior known single crystal superalloy casting compositions are their limited solid solution strength, their limited precipitation strength, due to low gamma prime content, and their narrow heat treatment "window" which limits the temperatures at which they can be manufactured or processed.